A liquid of low viscosity is required to couple high-intensity focused ultrasound (HIFU) transducers to the targeted tissue/organ, e.g., the abdomen, in both clinical and pre-clinical applications. A second, very thin layer of coupling medium, e.g., a gel, intervenes for direct contact with the skin of the patient.
The liquid, such as water, should be relatively pure, with a dissolved oxygen content less than or equal to three parts per million (PPM). Otherwise, the high-intensity focused field can easily create cavitation bubbles in the coupling fluid that could block and/or distort the ultrasound field. Inefficiencies in HIFU transducers are mainly in the form of heat they locally generate. The heat is continuously removed by the liquid coupling medium flowing laterally along and past the face, i.e., ultrasound interface surface, of the transducer.
Degassed water is also routinely used in the ultrasound transducer industry during transducer quality control and calibration procedures.
In one degassing method, the fluid passes degassed through a tube made of a semi-permeable material that allows only gases to permeate through the membrane, but not liquids. This is due to the tube's internal hydrophobic (i.e., water-repelling) coating. A vacuum is applied around the outer surface of the tube. The resulting pressure gradient draws the dissolved gasses out of solution. An example of this is found in U.S. Patent Publication No. 2005/0154309 to Etchells et al. (hereinafter “the Etchels publication”), the entire disclosure of which is incorporated herein by reference. Since the degassing chamber is in the shape of a tube, it lends itself nicely to closed-loop (i.e., inline) operation.
In another method, the fluid in a closed container is subjected to a vacuum. The vacuum lowers the boiling temperature, to remove (i.e., boil away) the dissolved gases of the fluid. As the fluid needs to be in the closed container under vacuum for some time, this approach does not lend itself to closed-loop/inline operation. Instead, these degassers will typically work in “batch” mode, wherein a certain amount of water will be ready for use after having been degassed for some time. A lot of user interaction is generally required to generate the degassed water, retain it, and transfer it to its final application.
Both approaches are able to degas water to a dissolved oxygen content less than or equal to one PPM quite easily, provided that the vacuum applied to either the outside of the semi-permeable tube (in the case of the hollow-filter cartridge) or to the closed container (in the case of batch degassers) is high. Typically, a pressure of between −27 and −29 inches of mercury (Hg) is required.